The use of helmet mounted displays (HMDs) in modern high-performance aircraft and rotorcraft is well known. The increasing complexity of these crafts has led to an increased burden on the pilot to visually interpret flight data from a large number of sources. The HMD helps to alleviate this burden by providing in the pilot's forward field of view a display of information essential for the pilot's performance of such tasks as target acquisition and weapon delivery. The HMD allows him to spend more time piloting the craft in a head-up mode, i.e., looking out at the exterior scene and not looking down as often at the instrument panel.
The information displayed by the HMD typically consists of symbols relating to pilotage and weapon targeting. This symbol information is fed by the onboard flight computer to a cathode ray tube (CRT) image source. The CRT image is then projected through a series of optical components, typically including partially reflective/ partially transmissive optical components located in front of the pilot's eyes. Viewing through the partially transparent components, sometimes referred to as a "combiner", the pilot is presented with a virtual image of the CRT image projected in his view of the external "real world" scene.
Depending on the ambient light conditions in which the pilot is flying, different requirements are placed on the optical design of the HMD. For the relatively high brightness daytime light (as compared to nighttime light), the combiner must have high transparency (see-through) since the pilot views the external scene as well as the projected symbol information. Consequently, the display source must have high brightness (e.g., a CRT written in the stroke mode) so as to produce enough contrast in the projected symbols.
For nighttime conditions, when flying by the "naked eye" is dangerous or impossible, night vision aids are required. In the present art, pilots utilize Image Intensifier devices employed in Night Vision Goggles (NVGs). An advantage of the HMD is that the nighttime viewing function can be accomplished with the HMD optical system. For such use, the external scene may be sensed by, for example, image intensified television of forward looking infrared devices. The output of these devices is electronically processed and fed to an image projection source such as a raster mode CRT. The processing may also include the addition of symbol data to the sensed image of the exterior scene. The resulting CRT image is projected in the pilot's forward field of view through the HMD projection optical components.
The raster mode CRT images are typically much lower in luminance than the visual symbol information produced by the stroke written CRT for the daytime situation. (Typical stroke written luminances can be 100 times as bright as typical raster mode luminances.) Thus, the highly transmissive, only partially reflective combiner used in a HMD designed for day usage is necessarily inefficient in transferring light from the CRT to the eye. (For some typical combiner designs, brightness transfer from the CRT to the eye can be less than ten percent). Consequently, the high see-through viewing optics commonly used in prior art HMD designs are not optimally suited for night flying conditions.
Another approach to night vision capability involves the direct incorporation on the helmet of image intensifier devices directly coupled to the HMD optics. Such apparatus is disclosed and claimed in a copending U.S. patent application of the same assignee entitled: DIRECT INCORPORATION OF NIGHT VISION IN A HELMET MOUNTED DISPLAY, U.S. Pat. application Ser. No. 313,686 filed on even date herewith by Fournier et al. There, the brightness levels provided by the image intensifiers are on the order of 1% or less of the typical CRT raster brightness, and the brightness transmitted to the eye from the high see-through viewing optics can be inadequate for normal vision, much less for piloting an aircraft.
The HMD prior art attempts to solve the luminance transfer problem by using a refractive relay system which uses only a single combiner (e.g., the IHADSS HMD from Honeywell). However, these HMDs introduce other problems, e.g., the diameters of the relay lenses tend to be large, and the eye relief (i.e., the distance from the observer's eye to the nearest HMD optical component) tends to be short. These dimensions are undesirable when attempting to design an HMD to meet the geometries imposed on the HMD by the human head.
Two completely different optical systems can be used to meet the requirements of both day and night viewing but this requires considerable extra hardware which is both costly and difficult to stow in the aircraft. It also raises some concerns about the changeover from one HMD to another when transitioning from daylight to nighttime. For example, when the pilot is on a mission that begins during daylight and runs through dusk into night, the pilot must replace the entire day HMD with the entire nighttime HMD. This changeover may occur at a critical time and may be so cumbersome as to require the pilot to land the aircraft to accomplish the changeover.